1. Field of the Invention
The present invention generally relates to an image forming apparatus such as a digital color copier, printer or the like, and, in particular, to a color-image forming apparatus in which color displacement does not occur, and, thereby, it is possible to form high-quality color images.
2. Description of the Related Art
Recently, in order to improve the image-formation efficiency, as disclosed in Japanese Laid-Open Patent Application No. 4-175774, in an image forming apparatus for forming multi-color images from image information obtained through an image reading device or sent from a host apparatus, two color images, each having a standard size, such as A-4 size, are formed on a photosensitive belt having a length more than twice that of the standard size, and two toner images formed on the photosensitive belt are transferred to transfer paper sheets which are fed successively. Alternatively, as disclosed in Japanese Laid-Open Patent Application No. 5-227386, a color image is formed on a first transfer paper sheet held by a transfer drum, which can hold a plurality of transfer paper sheets at a time, based on image data transmitted from an image reading device, and, then, color images are formed on second and following transfer paper sheets based on image data stored in an image memory.
However, when a color image is formed in a manner in which an image, the color of which is a first color (for example, black), is formed using image data transmitted from an image reading apparatus, and, then, images, colors of which are second, third and fourth colors (for example, cyan, magenta and yellow), are formed using image data stored in and read out from an image memory, so that these images are overlaid with each other, timing of a reference signal for positioning an image may be different between the first-color image and the second, third and fourth-color images. Thereby, color displacement may occur.